DESCRIPTION (adapted from the application) Hereditary hemochromatosis (HH) is a common autosomal recessive disorder of iron metabolism characterized by excess absorption of dietary iron. HH is caused by mutation of HFE, a MHC class I-like protein found in stable association with Beta-2-microglobulin and transferrin receptor in duodenal crypt cells. An essential role for HFE in regulating dietary iron absorption was confirmed by our observation that HFE knockout mice manifest excess iron loading. The mechanism by which HFE modulates intestinal iron absorption is unknown. Our working hypothesis is that disruption of HFE 1) decreases the uptake and/or increases the release of plasma iron by duodenal crypt cells, and thereby 2) decreases the crypt cell regulatory iron pool, which 3) increases apical expression of the iron transporter DMT1 in daughter enterocytes, which in turn, 4) increases dietary iron uptake. Our broad goal is to understand the mechanism by which HFE modulates dietary iron transport. We will pursue this goal by testing each step of the working hypothesis using two novel murine models-one in which the HFE gene has been disrupted, and the other in which it has been overexpressed. Two control groups with normal HFE alleles will be analyzed-one group iron-deficient and the other iron-replete. In the proposed studies on HFE knockout and overexpressing mice (and controls with normal alleles) we have 4 specific aims: 1) Measure the uptake and release of transferrin-bound and ionic iron by duodenal crypt cells. 2) Quantify iron-responsive element binding activity along the duodenal crypt-villus axis. 3) Characterize DMT1 mRNA and protein expression in duodenal enterocytes. 4) Measure duodenal iron uptake and transfer. These studies should improve our understanding of the pathogenesis of HH by defining the molecular events regulating dietary iron absorption and the consequences of functional loss of HFE. We hope the findings suggest novel approaches to prevent iron loading in HH and other diseases with excess dietary iron absorption.